Process for recovering pure aromatics



March 18, 1969 K. H. EISENLOHR ET 3,433,849

PROCESS FOR RECQVERING PURE AROMATICS Filed Aug. 2, 1966 TEMPERATURE "c.

INVENTOR.

KARL HEINZ EISENLOHR Y HANS WIRTH FRANCIS M. CRAWFORD United States Patent 8 Claims ABSTRACT OF THE DISCLOSURE The recovery of pure aromatic hydrocarbons from complex mixtures containing same, in the past, has been an involved. and complicated procedure. It has now been found according to the present invention that substantially pure fractions of benzene, toluene and xylene can be obtained from mixtures boiling within the range 70 to 150 C. and admixed with aliphatic saturated and unsaturated hydrocarbons and olefins, by subjecting said mixture to dual countercurrent solvent extraction with N- methylpyrrolidone containing 1525% by volume of water as a selective solvent and a liquid hydrocarbon fraction boiling below 60 C. as a counter solvent and an extract reflux to obtain a raflinate phase and an extract phase, and distilling said extract phase to separate said selective solvent and the extract containing benzene, toluene and xylene and aliphatic unsaturated hydrocarbons, and fractionating said extract to recover discrete pure fractions of benzene, toluene and xylene and collecting preliminary and transient fractions rich in aliphatic unsaturated hydrocarbons before and between said discrete pure fractions for recirculation as an extract reflux to said extraction process.

This application is a continuation-in-part of Ser. No. 167,871, filed Sept. 22, 1962, now abandoned.

The present invention relates to the recovery of aromatics in pure form from liquid mixtures of aromatic and nonaromatic hydrocarbons. More particularly, it relates to a process for the recovery of pure benzene, toluene and xylene by passing a liquid mixture of such aromatic hydrocarbons, and nonaromatic hydrocarbons having a boiling range between 70 and 150 C. through an extraction apparatus in countercurrent flow with a selective solvent consisting of aqueous N-methylpyrrolidone.

The simplest aromatic hydrocarbons having 6 to 8 C-atoms per molecule are benzene, toluene and xylene. These aromatic hydrocarbons are used as starting materials in numerous catalytic processes, as for example, in the production of terephthalic acid by catalytic oxidation of xylene; the production of cyclohexane by catalytic hydrogenation of benzene; or the production of benzoic acid by the catalytic oxidation of toluene. Such catalytic processes require the use of starting materials of a high degree of purity in order to prevent destruction of the catalysts used in the processes.

In general, it has not previously been possible to recover either aromatic hydrocarbons or nonaromatic hydrocarbons, such as naphthenes, parafilns or olefins from gasoline fractions or from similar complex mixtures of hydrocarbons by means of a simple fractional distillation, and, particularly, in a suflicient degree of purity for use in catalytic processes. A satisfactory separation of aromatic and nonaromatic hydrocarbons has previously been possible only by extraction by means of selective solvents. Many organic liquids have been suggested as selective solvents for this purpose, as, for example, sulfur dioxide, dethylene glycol, polyethylene glycols, polyethylene glycol ethers, etc. Other suitable selective solvents include furfural, carbonic esters, dimethyl formamide, monomethyl formamide, and cyclic sulfones, such as sulphones, butyrolacetone, pyrrolidones, or N-alkylated pyrrolidones. The selectivity of such solvents may be increased by the addition of water. With the abovementioned solvents, it is possible to obtain enriched fractions of aromatic hydrocarbons and nonaromatic hydrocarbons respectively comprising paraflins and naphthenes.

Countersolvents are sometimes used in such liquid extraction processes in order to increase the selectivity of the solvent. Suitable countersolvents for the extraction of aromatic and nonaromatic hydrocarbons includes paraffinic hydrocarbons having boiling ranges different from that of the mixture of aromatic and non-aromatic hydrocarbons being separated.

Instead of the countersolvent, a reflux of the extracted aromatic hydrocarbons may be recycled in the extraction process.

Countersolvents and reflux are often used simultaneously in liquid extraction processes.

The processes for the separation of aromatic and nonaromatic hydrocarbons referred to above have been used to recover kerosene from catalytic cycle oil by extracting the aromatic compounds therefrom or to recover unreacted naphthenes from the reactions of reforming processes.

These previously known processes involving extraction of mixtures of aromatic and aliphatic hydrocarbons with a selective solvent have not been practical for the recovery of aromatic hydrocarbons such as benzene, toluene and xylene in the high degree of purity required for catalytic processes. It has been found especially difiicult to obtain such compounds of the required degree of purity when the mixture being treated contains olefinic hydrocarbons, parafiins and naphthenes. When extracting such mixtures it has been found that only a portion of the olefins is collected in the nonaromatic raflinate while the remaining portion remains in the aromatic extract. Because of the fact that it has not been possible to separate all of the olefins from the aromatic hydrocarbons by extraction with a selective solvent followed by a subsequent fractionation, additional steps have been employed in order to reduce the olefin content of the starting mixture below the limits required for benzene, toluene and xylene to be used in catalytic processes. These required limits, in general, are less than 5 mol percent of olefins in the benzene, toluene or xylene. In order to obtain the aromatic hydrocarbon in this required degree of purity by the previously known extraction processes, it has been necessary to use a starting mixture containing only an extremely small quantity of olefins. In order to obtain starting materials with such extremely low olefins content it has been necessary to subject the starting mixture to a hydrogenation refining step prior to the extraction step.

In the hydrorefining of various starting materials rich in olefins, such as coke-oven benzene, cracked gasoline, or the like, and having a bromine number of the order of 10 to ml., the olefin content has customarily been reduced to less than 0.1 g./ 100 ml. By doing this it has been found possible to produce benzene having a bromine number less than 0. 0 1 g./ 100 ml. by extraction. In such cases a preferred subsequent treatment has comprised contacting the vaporized hydrocarbon with surface active materials, as, for example, with a bleaching agent, at a temperature of the order of 200 C.

It is an object of the present invention to provide an improved method for recovering pure benzene, toluene and xylene by extracting a liquid hydrocarbon fraction having a boiling range of the order of 70 to C. and comprising saturated and unsaturated aromatic and nonaromatic hydrocarbons, by subjecting such mixtures to countercurrent extraction with a selective solvent.

It is a further object of the present invention to provide a process for completely separating the olefins from aromatic extracts comprising benzene, toluene, xylene obtained in an extraction from a liquid mixture of saturated and unsaturated aromatic and nonaromatic hydrocarbons.

It has been discovered, in accordance with the present invention, that by using N-methylpyrrolidone as a selective solvent under specific conditions that it is possible to completely separate the olefins from aromatic compounds in a process for the extraction of benzene, toluene and xylene from mixtures thereof with nonaromatic hydrocarbons in a liquid hydrocarbon fraction having a boiling range of the order of 70 to 150 C.

It has been discovered that in the fractional distillation of an aromatic hydrocarbon extract containing even traces of olefins that the olefins tend to accumulate in the fractions having boiling ranges either below or between the boiling points of the pure aromatic hydrocarbons benzene, toluene and xylene. These conditions are illustrated in FIGURE 1, wherein the curve number 1, representing the distillation curve for benzene, toluene and xylene is drawn in a conventional manner so that in a continuous distillation the head temperature of the distillation is plotted against the quantity of the distillate.

Curve No. 2 is superimposed on curve No. 1 and shows the bromine number which indicates the olefin content of the individual fractions. The bromine number according to DIN 51,763 is used as the indication of the olefin content. The bromine number indicates the amount of bromine in grams (g.) which is absorbed in 100 ml. of an hydrocarbon mixture by addition to olefinic C C double bonds.

FIGURE 1 clearly shows that there is a high olefin content in the first runs of the distillation process and in the transitions between the distillations of the aromatic constituents thereof, namely benzene, toluene and xylene. By recirculating these intermediate fractions rich in olefins in the extraction process, the traces of olefins still present in the aromatic extract are returned as enriched fractions into the separation of aromatic and nonaromatic hydrocarbons and are thus added to the nonaromatic raffinate. When the process is carried out in a specific manner, the quantities of olefins still present in the aromatic extract are very small and, in general, less than 2%. In many cases these quantities are substantially smaller than 1%. Instead of returning into the extraction apparatus these fractions of the aromatic extract rich in olefins they can be added to the starting mixture in the extraction step, or even to the crude stock prior to the catalytic hydrogenation.

This separation of the olefins is so complete that the resulting pure products meet even the most stringent requirements for purity without the necessity of further treatment at high temperature with substances having a catalytic or a surface-active effect such as bleaching earth.

The process of the present invention for recovering pure benzene, toluene, and xylene from mixtures thereof with non-aromatic hydrocarbons comprises separating the aromatic and the nonaromatic hydrocarbons by extraction in countercurrent flow with a selective solvent for aromatic substances and a paraffinic countersolvent. The selective solvent used comprises a mixture of N-methylpyrrolidone with from to 25% by volume of water, the quantity of said solvent mixture being from 4 to 8 times the initial quantity of the aromatic substances in the starting mixture of aromatic and aliphatic hydrocarbons. The countercurrent solvent comprises a hydrocarbon fraction having a boiling point below 60 C. and preferably below 40 C. The quantity of countercurrent solvent used is from 0.2 to 0.8 times that of the initial quantity of the aromatic substances in the starting mixture of aromatic and nonaromatic hydrocarbons.

The hydrocarbon fraction used as the countercurrent solvent preferably contains propane, butane or pentane. It may, however, also contain olefins and naphthenes having boiling points in the same range as the hydrocarbons of the countercurrent solvent.

Suitable starting mixtures of aromatic and non-aromatic hydrocarbons for use in the present invention comprise all of the hydrocarbons obtained from the refining of solid or liquid hydrocarbon fuels. These substances particularly include coke-oven, benzene and gasoline obtained in the low-temperature carbonization as well as the reformed derivatives of petrol products and cracked gasolines obtained during the cracking of petrol fractions to olefins. The starting mixture can be purified by any known means, as, for exampe, by a catalytic hydrogenation refining to eliminate diolefins and to reduce any extremely high content of monoolefins.

In general, the process of the present invention is carried out at atmospheric pressure and within the temperature range of 10 to 40 C. If, for any particular reason it is desired to operate the process at a temperature at the upper limit of the required temperature range when a particularly low-boiling countercurrent solvent is used, the extraction process can be carried out under pressure and the extraction apparatus adjusted to an increased pressure.

The process of the present invention can be carried out in any conventional extraction tower or column but it is preferred to use a multistage mixer-settler extraction installation. The extraction apparatus is preferably constructed so that its effectiveness corresponds to 20 to 30 theoretical stages.

The starting liquid mixture of hydrocarbons is introduced into the extraction apparatus at a point between the eighth and eighteenth of the theoretical stages. Simultaneously, the selective solvent consisting of N- methylpyrrolidone and water is introduced in the first stage and the countersolvent is introduced in the last stage of the extraction apparatus.

During the extraction process carried out in accordance with the present invention the selective solvent separates the aromatic hydrocarbons as an extract from the starting mixture. The selective solvent and extract are discharged from the last stage of the extraction apparatus. The selective solvent is then separated from the aromatic extract by distillation and recirculated from the sump of the distillation column to the first stage of the extraction apparatus. The overhead product of this distillation is the aromatic extract still containing low boiling countersolvent and water. From this distillate the water is separated by stratification and recirculated with the selective solvent to the extraction apparatus. The nonaqueous layer of thi stratification is introduced into a fractionating distillation column and separated into a first fraction containing countersolvent, olefins, and some benzene, a second fraction consisting of pure benzene, a third fraction containing benzene, toluene, and olefins, a fourth fraction of pure toluene, a fifth fraction containing toluene, xylene and olefins, and a sixth fraction or sump product of pure xylene. The preliminary first fraction and the third and the fifth transition fractions containing olefins are recycled as an extract reflux in the last stage of the extraction apparatus.

The above extract reflux is obtained in a quantity of from 0.2 to 1.2 times the initial quantity of aromatic hydrocarbons in the starting mixture. When the sum of the olefin-containing fractions is smaller than the required reflux, a part of the crude extract is added. On the other hand, when the sum of the olefin-containing fractions is greater than the required refiux, the surplus is recycled to the starting mixture of the extraction process or even to the crude stock before the hydrogenating refining.

Other advantages of the process of the present invention will be apparent upon reference to the accompanying description when taken in conjunction with the attached drawings, wherein:

FIGURE 1 is a graph showing a distillation curve for the hydrocarbons and a second curve indicating the olefin content of the individual fractions.

FIGURE 2 is a schematic flow diagram of the extraction process of the present invention.

The process of the present invention will now be described in detail with particular reference to FIGURE 2. As shown in FIGURE 2, a mixture of hydrocarbons rich in olefins and containing aromatic hydrocarbons is introduced through conduit 1 into a hydrogenation refining stage 2, from which it is conducted into a distillation column 3. An appropriate fraction for the production of benzene, toluene and xylene, referred to as BTX (benzene-toluene-xylene) having a boiling range of 70 to 150 C. is introduced through conduit 4. The lower boiling point products of the hydrogenation are discharged from the top of the distillation column at 5 and the higher boiling point products from the sump at 6.

The fraction containing benzene, toluene and xylene (BTX-cut) is introduced into a multistage extraction apparatus 7 through the conduit 4 at an inner stage, e.g., between the eighth and eighteenth stages of a 30-stage extractor. Simultaneously therewith the selective solvent consisting of N-methylpyrrolidone and to 25% by volume of water is introduced into the first stage of the extraction apparatus through conduit 11. A countersolvent and the extract reflux are introduced into the last stage of the extraction apparatus through conduit 18.

The aromatic substances are dissolved in the selective solvent during the countercurrent flow through the extraction apparatus 7. The extract dissolved in the selective solvent is discharged from the extraction ap aratus through conduit 8 into a distillation column 10 inwhich the aromatic extract is separated as a top fraction from the selective solvent which is removed as a sump product from the column 10 and is then recirculated to the extraction apparatus through conduit 11.

The non-aromatic constituents of the starting mixture forming the raflinate are removed from the extraction apparatus together with the low boiling countersolvent through conduit 9 and are introduced into a distillation column 12. The rafiinate comprising the nonaromatic substances of the starting mixture accumulate as a sump product in the distillation column 12 and are removed therefrom through conduit 13. The low boiling countersolvent is returned into the last stage of the extraction apparatus through conduits 14 and 18. 1'

The overhead product of the distillation column 10, forming the crude extract, is separated in a stratification separator (not shown) into an aqueous layer and a hydrocarbon layer. While the aqueous layer is returned to the selective solvent, the dry extract is introduced into a fractionating distillation column 17.thr0ugh conduit 16. A portion of this dry aromatic extract may be returned directly to the extraction apparatus 7 through conduit 15. It is preferable, however, to introduce the entire quantity of the dry aromatic extract into the fractionating columns 17 from which pure benzene is removed as a side fraction through conduit 20. The first runs, rich in olefins, accumulate in the head of the column 17 and are withdrawn as an overhead distillate and recirculated through lines 19 and 18 as an extract reflux to the last stage of the extraction apparatus 7.

The sump product of column 17 containing the aromatic substances having boiling points higher than that of benzene are introduced into a further fractionating distillation column 22 through conduit 21. Pure toluene is removed as a side fraction through conduit 24 from column 22. An intermediate fraction containing benzene and toluene having a comparatively high olefin content is withdrawn as a top product from the head of the column 22 which fraction is mixed with the head prodnot from column 17 and returned to the extraction apparatus 7 as an extract reflux through conduit 19 and 18. As indicated by the dotted line 26, a part of these mixed head products may be added to the starting mixture in line 4. On the other hand, a part of the crude extract, forming the head product of column 10, may be recycled as an extract reflux to the extraction apparatus as indicated by dotted line 15.

A higher boiling fraction of the aromatic extract comprising xylene accumulates in the sump of distillation column 22. This sump product can be separated in further distillation steps which are not described or illustrated but which are similar to that described in respect to benzene and toluene (columns 17 and 22). The top product of this further distillation is a mixture of toluene and xylene relatively rich in olefins and is mixed with the other overhead fractions containing olefins from the fractionating column 17 and 22 as an extract reflux. Pure xylene is recovered as a side fraction. Aromatic hydrocarbons having higher boiling points than xylene are recovered from the sump.

The following specific examples are given to further illustrate the invention disclosed above. It should be borne in mind, however, that this example is illustrative only and in no way is to be considered as limiting the invention. Any variations therefrom which are obvious to one skilled in the art and which do not depart from the basic concept described herein are intended to come within the scope of the appended claims.

EXAMPLE I Gasoline obtained by pyrolysis of a hydrocarbon mixture was introduced through conduit 1 into the hydrogenation prerefining stage 2 from which was discharged a prerefined gasoline at a rate of 1000 kg./h. and having the following composition:

Kg./h. Nonaromatic hydrocarbons boiling below C. 196 Benzene 188 Toluene 168 Xylene 87 Nonaromatic substances boiling within the range from 70 to 150 C. 261 Residue boiling above 150 C. and consisting of nonaromatic substances -1 Benzene 184 Toluene 168 Xylene 83 Nonaromatic substances boiling at from 70 to The above mixture constituted the starting mixture for the extraction process.

200 kg. of an overhead product consisting of 196 kg. of nonaromatic hydrocarbons having a boiling range of below 70 C. and 4 kg. of benzene were withdrawn as a byproduct from distillation column 3 through conduit 5.

From the sump of column 3, 100 kg. of a residue boiling above 150 C. and consisting of nonaromatic and higher aromatic substances, and 4 kg. of xylene were withdrawn through conduit 6.

The starting mixture containing benzene, toluene and xylene was withdrawn from distillation column 3 through conduit 4 and was introduced into approximately the th stage (between the 8th and 18th stages) of an extratcion apparatus comprising stages of a mixer-settler arrangement and at a temperature of 20 C. The extraction apparatus was operated at atmospheric pressure.

Simultaneously with the introduction of the starting mixture into the extraction apparatus, 3580 kg. of the selective solvent comprising N-methylpyrrolidone (density: 1.03) and a water content of 15% by volume were introduced through line 11 into the first stage of the extraction apparatus at a temperature of C. At the same time a countersolvent consisting of 161 kg. of a hydrocarbon fraction boiling below C. and consisting substantially of pentane was introduced through conduit 18 into the last stage of the extraction apparatus at a temperature of 10 C. The head product of the distillation column 17, consisting of 120 kg. of benzene and 123 kg. of pentane and the head product of distillation column 22, consisting of 15 kg. of benzene, 25 kg. of toluene and 0.5 kg. of nonaromatic substances, essentially olefinic substances boiling at from 70 to 150 C. were introduced as an extract reflux into the extraction apparatus 7 at the same time through conduit 18.

The extract phase of the composition shown below was discharged from the extraction apparatus 7 through conduit 8:

Kg. Pentane 123 Benzene 3 19 Toluene 193 Xylene 83 Nonaromatic substances boiling at 70-150 C. 0.5 N-methylpyrroliidone 3580 The 3580 kg. of selective solvent (N-methylpyrrolidone and water) were completely separated in the distillation step in column 10 from the extract and returned through conduit 11 for recirculation in the extraction apparatus 7.

422 kg. of the rafiinate phase were withdrawn from the extraction apparatus through conduit 9 and found to consist of 161 kg. of countersolvent (pentane) and 261 kg. of nonaromatic substances boiling at from 70 to 150 C. This raifinate phase was then introduced through line 9 into the distillation column 12, from which 261 kg. of nonaromatic substances boiling at from 70 to 150 C. were withdrawn as a sump product through conduit 13. 161 kg. of countersolvent (pentane) were withdrawn as a top product from the head of distillation column 12 and returned through conduits 14 and 18 for recirculation in the extraction apparatus.

Crude aromatic extract was withdrawn as a top product from the head of the distillation column 10 and, after separation of water by Stratification, found to consist of:

Nonaromatics having a boiling range of 70 to The above crude aromatic extract was introduced through conduit 16 to the first fractionating distillation column 17.

123 kg. of pentane and 120 kg. of benzene were distilled from the head of the column 17. This overhead distillate, rich in olefins, was returned through conduits 19 and 18 into the last stage of the extraction apparatus as an extract reflux.

184 kg. of benzene, having a melting point of C. were withdrawn as a side product from column 17.

251 kg. of sump product, consisting of 100 kg. of toluene and 83 kg. of xylene, were withdrawn from column 17 through conduit 21 and separated in the following distillation column 22 into toluene and xylene. The pure toluene obtained as a side fraction from conduit 24 had an index of refraction of 1.4967 at a temperature of 20 C. A similar process is set out in somewhat greater detail in the example given below.

EXAMPLE II A gasoline obtained by pyrolysis was introduced through conduit 1 into the hydrogenation prerefining stage 2 from which 1000 kg./h. of a prerefined gasoline was produced, having the following composition:

Kg./h.

First run nonaromatic hydrocarbons having a boiling range below C. 196 Benzene 188 Xylene 87 Nonaromatic substances boiling between 70 and 150 C 261 Residue boiling above 150 C., consisting of nonaromatics and higher aromatic substances This prerefined gasoline was introduced through line 4 into a distillation column having 40 trays.

As a side fraction from the distillation column there was withdrawn through conduit 4 a fraction having a boiling range from 70 to 150 C., ordinarily referred to as a BTX cut, and having the following composition:

The bromine number of this cut, used as the starting material for the extraction process, was 10 g./100 ml.

200 kg. of a top product consisting of 196 kg. of nonaromatic hydrocarbons boiling below 70 C. and 4 kg. of benzene were withdrawn from column 3 through conduit 5. 104 kg. of a residue boiling above 150 C. and consisting of 100 kg. of nonaromatics and higher aromatic substances and 4 kg. of xylene were discharged from the sump of column 3 through conduit 6.

The BTX-cut, used as a starting mixture, was introduced through conduit 4 into the 15th stage (between the 8th and 18th stages) of a mixer-settler extraction apparatus comprising 30 stages. This starting mixture was at a temperature of 20 C. Simultaneously, 3580 kg. of the selective solvent consisting of N-methylpyrrolidone and having a water content of 15% and a temperature of 40 C. was introduced through conduit 11 into the first stage of the extraction apparatus 7 operated under atmospheric pressure. Simultaneously, a countersolvent consisting of 161 kg. of a hydrocarbon fraction boiling below 50 C. and containing substantially pentane was introduced through conduit 18 into the last stage of the extraction apparatus at a temperature of 10 C., together with the head product of the distillation column 17 consisting of kg. of benzene and 163 kg. of pentane, and the head product of column 22 consisting of 15 kg. of benzene, 25 kg. of toluene and 0.5 kg. of nonaromatic rich in olefins boiling between 70 and C.

The extract phase discharged from the extraction apparatus 7 through conduit 8 and introduced into the distillation column 10 consisted essentially of 123 kg. of pentane, 319 kg. of benzene, 193 kg. of toluene, 83 kg. of xylene, and 0.5 kg. of nonaromatic substances rich in olefins. and boiling between 70 and 150 C., 3580 kg. of N-methylpyrrolidone and 15% by volume of water. The entire 3580 kg. of selective solvent were recirculated to the extraction apparatus through the conduit 11.

422 kg. of the raflinate phase, containing 261 kg. of nonaromatic constituents of the starting mixture, together with 161 kg. of the countersolvent pentane, were conducted from the extraction apparatus 7 through conduit 9 and introduced into the distillation column 12. 261 kg. of nonaromatic substances boiling between 70 and 150 C. were withdrawn from the sump of column 12 through the conduit 13 and had a bromine number of 26 g./100 ml. 161 kg. of countersolvent (pentane) were withdrawn as a top product from the distillation column 12 and returned into the extraction apparatus through conduits 14 and 18.

The crude extract was withdrawn as an overhead fraction from the distillation column 10. After the separation of water by stratification, the raw extract was found to consist of: 123 kg. of pentane, 319 kg. of benzene, 193 kg. of toluene, 83 kg. of xylene, 0.5 kg. of nonaromatic substances rich in olefins and boiling between 70 and 150 C. This crude extract had a bromine number of 0.5 g./ 100 ml. It was introduced through conduit 16 into the first fractionating column 17.

From the head of the column 17, 123 kg. of pentane and 120 kg. of benzene were distilled off. This distillate, which had a bromine number of 1.2 g./ 100 ml. and was relatively rich in olefins, was returned as an extract reflux through conduits 19 and 18 into the last stage of the extraction apparatus. 184 kg. of pure benzene having a melting point of 55 C. and a bromine number of less than 0.005 g./100 ml., corresponding to a purity of 99.98%, were removed as a side product from column 17 through line 20.

291 kg. of sump product were withdrawn from the sump of distillation column 17 through conduit 21 and further rectified in column 22. This residue of column 17 consisted of 15 kg. of benzene, 193 kg. of toluene, 83 kg. of xylene, and 0.5 kg. of aromatic substances boiling between 70 and 150 C. and had a bromine number of 0.2 g. Br/l ml.

The head product of column 22 is an intermediate cut between benzene and toluene and consists of kg. of benzene, 25 kg. of toluene and 0.5 kg. of nonaromatic substances boiling between 70 and 150 C. It has a bromine number of 1 lg. -Br/100 ml. and was relatively rich in olefins. It was returned into the extraction apparatus 7 through conduits 27, 19 and 18, as an extract reflux.

116 kg. of pure toluene were removed through conduit 24 as a side fraction from column 22. This toluene had an index of refraction of 1.4967 at 20 C. and a bromine number of les than 0.01 g./ 100 ml., corresponding to a purity of 99.9% toluene.

2 kg. of toluene and 83 kg. of xylene were removed from the sump of column 22 through conduit 25. If desired, this fraction could be processed to obtain pure xylene.

The composite top products from columns 17 and 22 had had a bromine number of 1.17 g. Br/l00 ml. This value shows that the olefins from the starting mixture dissolved in the extract phase are markedly enriched in the preliminary fraction and in the transition fractions between the fractions of pure benzene and pure toluene. By recirculating the overhead products from columns 17 and 22 in the extraction process, the olefins from these concentrates are directed to the raffinate phase.

From the above results it is evident that by the process of the present invention it is possible to obtain benzene, toluene and xylene in a high degree of purity in a much simpler manner than has previously been possible by the prior art methods. These pure products are now obtainable only by an extraction process followed by a fractionation. Subsequent treatment of the aromatic extract, or its fractions, at elevated temperatures of the order of 200 C., are no longer required in order to obtain materials of a high degree of purity.

In view of the complete separation of the olefins from the aromatic extract by the process of the present invention, it is now possible to substantially simplify the usual pretreatment processes of the crude materials containing benzene, toluene and xylene. This is particularly true 1n the case of catalytic hydrogenation. Since the starting mixtures for the extraction step in the process can now contain larger amounts of olefins, milder reaction conditions can be used for the hydrogenation refining the crude starting materials. This results in a reduced hydrogenation effect upon the aromatic hydrocarbons, and accordingly in higher yields of the desired materials. In addition, it is now possible to use in the hydrogenation step a gas of lower hydrogen partial pressure or to use catalysts having a greater desul-furization effect but a lower hydrogenation effect. It is thus possible to carry out the hydrogenation step under more economical operating conditions.

What is claimed is:

1. In a process for recovering benzene, toluene and xylene in a high degree of purity suitable for catalytic purposes from a hydrocarbon fraction having a boiling range of from 70 to 150 C. and containing saturated and unsaturated aromatic and aliphatic hydrocarbons by countercurrent extraction with a selective solvent and a countersolvent in a multistage extractor comprising the steps: introducing said mixture in an inner stage of said multistage extraction, introducing N-methylpyrrolidone containing 15-25% by volume of water as a selective solvent in the first stage of said multistage extractor, introducing a liquid hydrocarbon fraction boiling below 60 C. as a countersolvent in the last stage of said multistage extractor, discharging a raffinate phase from the first stage of said multistage extractor, distilling said raffinate phase to recover said countersolvent as a distillate for recycling in the last stage of said multistage extractor and the aliphatic hydrocarbons as a sump product, discharging an extract phase from the last stage of said multistage extractor, distilling said extract phase to separate a hydrocarbon extract and water as a distillate and the selective solvent as a sump product, separating the water from said hydrocarbon extract by Stratification, recycling said selective solvent and said water to the first stage of said multistage extractor, distilling said hydrocarbon extract in a fractionating column to recover discrete fractions of pure benzene, toluene, xylene, collecting preliminary and transition fractions containing olefins before said pure fractions of benzene, toluene and xylene for recirculation as an extract reflux to the last stage of said multistage extractor.

2. In a process for recovering benzene, toluene and xylene in a high degree of purity suitable for catalytic purposes from a hydrocarbon mixture having a boiling range from 70 to C. containing saturated and unsaturated aromatic and aliphatic hydrocarbons as a starting mixture by countercurrent extraction in a multistage extractor with a selective solvent and a counter solvent to produce an extract phase containing aromatic hydrocarbons and the selective solvent and a raflinate phase containing aliphatic hydrocarbons and the counter solvent, comprising the steps of introducing the starting mixture in an inner stage of said multistage extractor, introducing N-methylpyrrolidone with l5%-25% per volume of water as a selective solvent in the first stage of said multistage reactor, the amount of said selective solvent being 4 to 8 times by volume the quantity by volume of the aromatic hydrocarbons in said starting mixture, introducing a liquid hydrocarbon fraction boiling below 60 as a counter solvent in the last stage of said multi-stage extractor, the amount of said counter solvent being 0.2 to 0.8 times by volume the quantity by volume of the aromatic hydrocarbons in the starting mixture, distilling the raffinate phase discharged from the first stage of said multistage extractor to remove the countersolvent for recirculation in the extraction step, distilling the extract phase discharged from the last stage of the multistage extractor to separate the selective solvent and water for recirculation in the extraction step and aromatic extract, distilling said aromatic extract in a fractionating column, recovering discrete fractions of pure benzene, toluene and xylene, collecting preliminary or transition fractions containing olefins before said discrete fractions of pure benzene, toluene, and xylene for recycling as an extract reflux in the last stage of said multistage extractor, the amount of said extract reflux being 0.2 to 1.2 times by volume the quantity by volume of the aromatic hydrocarbons in the starting mixture.

3. A process according to claim 2 wherein the surplus of the collected preliminary and transition fractions over the amount to be recirculated as an extract reflux is added to the starting mixture.

4. A process according to claim 2 wherein a difference between the collected preliminary and transition fractions and the amount to be recirculated as an extract reflux is completed by a part of the raw aromatic extract.

5. A process according to claim 1, the temperature in the extraction step being 10 to 40 C., the temperature in the last stage being lower than in the first stage.

6. In a process for recovering aromatic hydrocarbons from their mixture with nonaromatic hydrocarbons by countercurrent extraction With a selective solvent and a low boiling hydrocarbon countersolvent to obtain a raffinate phase containing the nonaromatic hydrocarbons and the counter solvent and an extract phase containing the aromatic hydrocarbons and the selective solvent and separating said aromatic hydrocarbons and said selective solvent by distillation, the improvement which comprises recovering benzene, toluene and xylene in a high degree of purity suitable for catalytic processes from their mixture with aliphatic saturated and unsaturated hydrocarbons in a fraction having a boiling range from 70 to 150 C. by extracting said mixture with methylpyrrolidone containing 15 to 25% by volume of water as a selective solvent and a liquid hydrocarbon fraction boiling below 60 C. as a counter solvent, distilling the extract phase from said extraction step to separate the selective solvent and the extract containing benzene, toluene and xylene and aliphatic unsaturated hydrocarbons and fractionating said extract to recover discrete pure fractions of benzene, toluene and xylene, and collecting preliminary and transition fractions rich in aliphatic unsaturated hydrocarbons before and between said discrete pure fractions for recirculation as an extract reflux in said extraction step.

7. The process of claim 6 wherein the said selective solvent is used in amount ranging from 4 to 8 times the volume of the aromatic hydrocarbons in the starting mixture and said counter solvent is used in amounts ranging from 0.2 to 0.8 times the volume of the aromatic hydrocarbons in said starting mixture.

8. The process of claim 6 wherein the temperature in the extraction step ranges from 10 to 40 C. and the temperature in the last stage is lower than that of the first stage.

References Cited UNITED STATES PATENTS 2,737,538 3/1956 Nelson 260-674 2,799,627 7/ 1957 Haensel 20896 2,840,620 6/1958 Gerhold et a1. 20896 2,886,610 5/1959 Georgian 260674 2,933,448 4/1960 Morin et a1. 260-674 2,956,005 10/ 1960 Lutz et a1. 20896 FOREIGN PATENTS 812,114 4/ 1959 Great Britain.

DELBERT E. GANTZ, Primary Examiner.

C. E. SPRESSER, JR., Assistant Examiner.

US. Cl. X.R. 208-96, 325, 326 

